“Survival of the fittest” is not a moral law. It’s the scientific observation that, for populations of organisms that reproduce and make mistakes while reproducing, those mistakes that better fit the environment the organisms are in are more likely to be passed on, resulting in, generations later, those populations eventually adapting to better fit their environment. There’s no obligation to consider and follow it as if handed down by a judge in the same way there’s no obligation to consider and follow gravity when walking outside.